Rotating assemblies for turbomachinery are often held together using an axial compressive load applied via a tie-shaft passing through the center of the rotating assembly and a nut threaded onto the end of the tie-shaft. These rotating assemblies must be balanced for use in high speed machinery and the required level of precision can be highly sensitive to the positions of the components.
Gas turbine engines include rotating components such as a fan, a compressor, a shaft, a seal and a turbine. A nut is often used on the end of a threaded shaft to secure and position one or more engine components relative to the shaft. The shaft typically has a radial flange extending outward at one end to provide an abutting surface and threads for the nut at the opposite end. The rotating engine components are stacked along the shaft such that the shaft extends through the center of the components. The nut is threaded to the shaft to apply an axial compressive force through the components that secures them in place relative to the shaft, and thus pilots the components.
Components in a rotating group require an axial facing pilot and a radially oriented pilot when mated to another component. The threads of a nut and bolt (or tie-shaft) provide both an axial facing pilot and a radially oriented pilot at the nut to tie-shaft interface.
The axial facing pilot and radially oriented pilot require geometric control such that these features are true to each other (perpendicular). Lack of perpendicularity of the axial facing pilot and radially oriented pilot results in shaft bow. It is straightforward to control the perpendicularity between the face and the inner or outer diameter surfaces of a component; however, it is difficult to have precision control between the threads of a nut and the face of the nut. This is also true of a bolt, tie-shaft, or other threaded component(s).
For instance, when the nut is threaded onto the shaft and the rotating assembly is placed under axial clamp load, the radial position of the nut can be driven off of the desired centerline position and could be inconsistent from build to build, causing unrepeatable balance results and associated vibration effects. This adversely affects the ability to make consistent, repeatable balance corrections to the rotor assembly. The inclusion of a radial piloting surface on the nut provides a feature to prevent radial movement of the nut and provide consistent positioning of the nut relative to the rotating assembly. This can be accomplished via a piloting surface on an inner diameter or outer diameter surface of the nut. A drawback of placing the piloting feature on an inner diameter surface of the nut is that, under axial load, the nut can expand radially outward thus loosening the piloting fit of the inner surface and negating the benefit of this feature for the balancing process. The outer diameter pilot will increase in size when the nut is loaded, thus maintaining the desired piloting effect and is therefore considered superior to the inner diameter pilot. Various conventional designs for the tie-shaft and nut have been proposed and used in gas turbine engines to maintain position control of the nut relative to the rotor stack, but improvements are still needed in the art.